In digital television (DTV) receivers there is a demand to reject interferences at the third order and the fifth order of the desired DTV frequencies, which are introduced, for instance, by wireless applications in accordance to the standards of communication networks of the second generation (2G) and/or third generation (3G). One setback is caused by the fact that these third order harmonics and fifth order harmonics of the frequencies in question cannot be completely filtered out by means of RF filters. Therefore, a harmonic-rejection mixer is necessary for coexistence.
It was figured out that one critical specification of a harmonic rejection mixer is located in the relative time shift in the components constituting the switching control signal waveform, as for instance, described by H. Brekelmans and L. Tripodi in “Pre-study result RF tuner for TV on Mobile in CMOS90”, Technical Note PR-TN 2005/01139, February 2006. In FIG. 4A, the time shift Td, i.e. the relative time shift between a rooftop and a main switching waveform which combine to one switching control signal, is illustrated. Further, as depicted in FIG. 4B, the time shift Td must substantially not exceed 4 psec to maintain 40 dB of rejection. Accordingly, with respect to a period of T=1.25 nsec at 800 MHz, only error of 0.32% is acceptable.
Therefore, it is an object of the invention to provide an improved method and apparatus for generation of a LO frequency in a harmonic-rejection receiver.
It is yet another object of the invention to provide an improved method and apparatus for generation of a LO frequency, by which systematic phase errors can be avoided, thus additional calibration means may be eliminated. The invention is defined by the independent claims. Dependent claims describe advantageous embodiments.